英文:
How to use Comparable correctly with Dart 3?
问题
在Dart 3中，您不能再将任何类用作mixin，也不能将接口类扩展到其库之外。为了迁移到Dart 3，您可以采用以下方法：
首先，移除mixin中的on Comparable<T>部分，因为Dart 3不再支持这样的用法。然后，将您的类改为实现Comparable<T>接口，而不是继承它。
下面是迁移后的代码示例：
mixin Compare<T> {
bool operator <=(T other) => compareTo(other) <= 0;
bool operator >=(T other) => compareTo(other) >= 0;
bool operator <(T other) => compareTo(other) < 0;
bool operator >(T other) => compareTo(other) > 0;
}
class Foo implements Comparable<Foo> with Compare<Foo> {
final int value;
const Foo({required this.value});
@override
int compareTo(Foo other) => value.compareTo(other.value);
}
通过这种方式，您不再将Comparable<Foo>用作mixin，而是将其用作接口，并且现在可以在Dart 3中正常工作了。
英文:
With Dart SDK 2.x.x. it was okay to use any class as mixin. But with Dart 3 this is no longer allowed as stated in the documentation: https://dart.dev/resources/dart-3-migration#mixin
Now I struggle with the migration. For example, I used the Comparable interface as mixin like this:
mixin Compare<T> on Comparable<T> {
bool operator <=(T other) => compareTo(other) <= 0;
bool operator >=(T other) => compareTo(other) >= 0;
bool operator <(T other) => compareTo(other) < 0;
bool operator >(T other) => compareTo(other) > 0;
}
class Foo with Comparable<Foo>, Compare<Foo> {
final int value;
const Foo({required this.value});
@override
int compareTo(Foo other) => value.compareTo(other.value);
}
But this gives me the error error: The class 'Comparable' can't be used as a mixin because it's neither a mixin class nor a mixin.
But if I try to use it as interface like this
mixin Compare<T> on Comparable<T> {
bool operator <=(T other) => compareTo(other) <= 0;
bool operator >=(T other) => compareTo(other) >= 0;
bool operator <(T other) => compareTo(other) < 0;
bool operator >(T other) => compareTo(other) > 0;
}
class Foo extends Comparable<Foo> with Compare<Foo> {
final int value;
const Foo({required this.value});
@override
int compareTo(Foo other) => value.compareTo(other.value);
}
I get another error The class 'Comparable' can't be extended outside of its library because it's an interface class.
So what is the best way to migrate my code to Dart 3?
答案1
得分: 1
以下是您要翻译的内容：
"你所试图实现的可以通过扩展来实现。
void main() {
final a = Foo(value: 20);
final b = Foo(value: 40);
print(a > b);
}
extension Compare<T> on Comparable<T> {
bool operator <=(T other) => compareTo(other) <= 0;
bool operator >=(T other) => compareTo(other) >= 0;
bool operator <(T other) => compareTo(other) < 0;
bool operator >(T other) => compareTo(other) > 0;
}
class Foo implements Comparable<Foo> {
final int value;
const Foo({required this.value});
@override
int compareTo(Foo other) => value.compareTo(other.value);
}
如果您注释掉扩展代码，那么对于 print(a > b);，您将在编译时出现错误，因此根据Dart3，这是有效的代码。"
英文:
What you were trying to get can be achieved by extensions.
void main() {
final a = Foo(value: 20);
final b = Foo(value: 40);
print(a > b);
}
extension Compare<T> on Comparable<T> {
bool operator <=(T other) => compareTo(other) <= 0;
bool operator >=(T other) => compareTo(other) >= 0;
bool operator <(T other) => compareTo(other) < 0;
bool operator >(T other) => compareTo(other) > 0;
}
class Foo implements Comparable<Foo> {
final int value;
const Foo({required this.value});
@override
int compareTo(Foo other) => value.compareTo(other.value);
}
If you comment extension code, you will get compile time error for print(a >b); hence this is valid code as per Dart3
答案2
得分: 1
@jamesdlin的评论回答了我的问题：
mixin Compare<T> implements Comparable<T> {
bool operator <=(T other) => compareTo(other) <= 0;
bool operator >=(T other) => compareTo(other) >= 0;
bool operator <(T other) => compareTo(other) < 0;
bool operator >(T other) => compareTo(other) > 0;
}
class Foo with Compare<Foo> {
final int value;
const Foo({required this.value});
@override
int compareTo(Foo other) => value.compareTo(other.value);
}
这就是为什么Compare可以是一个mixin。
英文:
The comment of @jamesdlin answers my question:
mixin Compare<T> implements Comparable<T> {
bool operator <=(T other) => compareTo(other) <= 0;
bool operator >=(T other) => compareTo(other) >= 0;
bool operator <(T other) => compareTo(other) < 0;
bool operator >(T other) => compareTo(other) > 0;
}
class Foo with Compare<Foo> {
final int value;
const Foo({required this.value});
@override
int compareTo(Foo other) => value.compareTo(other.value);
}
That why, Compare can be a mixin.
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